Various filtration devices are known to filter polymers, e.g. molten or dissolved polymers, for example prior to spinning filaments from the molten or dissolved polymer using a spin pack or spinneret.
The filtration devices for polymer filtration usually comprise filter candles, also named candle filters or tubular filters.
A filter candle apparatus for polymer filtration is disclosed in U.S. Pat. No. 5,279,733. The filter candles comprise small interstices between different paths of the filter candle, e.g. small interstices between the bearing and the mesh pack end fitting or in the small interstices between the core tube and the bearing and end fitting.
The filter candle, which is to filter polymer “outside-in”, has some disadvantages. Usually, high pressure is used during filtration, e.g. process pressures in the range of 200 bar to 500 bar, and using pressure drops over the filter candle of up to 150 bar, which pressures are necessary due to the high viscosity of the polymer fluid, e.g. about 100 Pa*s to 1000 Pa*s. Due to this pressure, some polymer is forced to flow in the small interstices between the bearing and the mesh pack end fitting or in the small interstices between the core tube and the bearing and end fitting. The polymer, which is pressed in the interstices, will no longer flow and harden, causing the polymer to become brittle, usually blackened. After some time, the brittle polymer will loosen and create hard polymer particles in the polymer fluid. The interstices are located at the clean, filtrate side of the filter membrane. So the polymer particles, which are created at these interstices, will flow along with the filtered polymer, and may cause problems in the further process, which uses the filtered polymer. E.g. it may cause spinnerets to be partially clogged, or it may create black contamination in polymer articles using the filtered polymer.
One way to at least partially avoid this black contamination due to hardened polymer particles is the use of a filter candle as disclosed in EP-A1-1872839. A ring-shaped seal is provided between the outer end surface of the core tube and the inner end surface and the end flange of the end fitting, which seal is a porous seal structure, whose pores having an average pore size equal or less than the filter rating of the filter medium of the mesh pack. The provision of such porous seals is expensive, in particular since the seals, made from expensive metal powders or fibres, are often disposed after a single use.